In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is eventually transferred to the surface of a material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in response to signals from a digital copy of the image in a computer connected to a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
In general, lithographic printing plates contain a colorant (dye or pigment) in the radiation-sensitive composition that has the function of making the image visible in order to be evaluated by optical density measurements before being mounted on press. In other words, such colorants provide contrast between the image and the background. Certain lithographic printing plates cannot contain a colorant for different reasons. For example, the imaged printing plate precursors that are usually developed on-press have a colorless coating because if a colorant is present, it could contaminate the lithographic printing ink and the fountain solution used for development and printing, with the result of altering the printed color shades. However, sometimes it is necessary for such printing plates to be used the same way as those developed off-press. In such instances, the image needs to be seen and evaluated.
Other lithographic printing plates contain materials that are not compatible with contrast-providing colorants. Such printing plates have a faint colored image that is difficult to distinguish from the anodized aluminum substrate background. This low image contrast makes it almost impossible to evaluate these printing plates for image quality, such as image resolution as measured using optical density measurements before mounting the plates onto a press. Such “colorless” plates are also difficult to “register” (align) when mounting them onto a press. Image registration (alignment) is very important in color printing in order to ensure image sharpness (resolution) and correct tinting shades.
U.S. Pat. No. 6,451,491 (Dhillon et al.) describes the high loading of contrast-providing pigments into the imaging layer using specific poly(vinyl acetal) polymers and specific combinations of loading solvent mixtures. Such high amounts of pigments may not be suitable for all lithographic printing plates as they can destabilize imaging chemistry or developers used to remove non-imaged regions in negative-working lithographic printing plate precursors.
Other contrast-providing colorants are obtained from leuco dyes that become colored in the presence of an acid or thermal acid generator, as described for example, in U.S. Pat. No. 7,402,374 (Oohashi et al.), U.S. Pat. No. 7,425,406 (Oshima et al.) and U.S. Pat. No. 7,462,440 (Yamasaki). These imaging materials have some disadvantages: (i) the acid or radical forming mechanism can be triggered prematurely during the drying of the plate leading to un-wanted color, especially in on-press developed printing plates, (ii) many times, the colored form of the dye is a salt that is soluble in water leading to excessive coating loss if the plate is developed in developer, in severe instances the coating and the color being washed off in developer, (iii) some of the colored forms of the dyes, especially the tri-aryl methane ones are light sensitive introducing additional precautions in handling the plates, and (iv) a high percentage of the leuco dye and of the latent acid has to be added to the coating to ensure a good contrast between the image and background. These components adversely affect the shelf-life, the mechanical properties, and the run-length of the photosensitive coating.
There is a need for an improved means for providing contrast between the image and background of lithographic printing plates, especially those prepared from negative-working lithographic printing plate precursors.